It is well known that an ultrasonic signal of sufficiently high intensity, amplitude-modulated with an audio signal, will be demodulated on passage through the atmosphere, as a result of a non-linear propagation characteristics of the propagation medium. Prior systems based on this phenomenon have been used to project sounds from a modulated ultrasonic generator to other locations from which the sounds appear to emanate. Specifically, arrays of ultrasonic transducers have been proposed for projecting audio-modulated ultrasonic beams, which can be steered to move the locations of the apparent sources of the demodulated audio contents. Moreover, the audio signals regenerated along the path of the ultrasonic beam are characterized by directivity corresponding to that of the beam. The signals can thus be directed to a particular location, with the audio signals being received at that location and not at other locations disposed away from the beam axis.
The directivity of the audio signals is maintained when the ultrasonic beam is reflected from a surface and, in fact, a proposed beam steering arrangement involves the use of a rotatable reflecting surface. On the other hand, if the beam is projected to a surface that absorbs acoustical energy at ultrasonic frequencies but reflects it at audio frequencies, the audio content of the signal will be reflected with reduced directivity, with the sound appearing to originate at the point of reflection. These characteristics give rise to a number of highly useful applications of these systems. For example, one may direct the ultrasonic beam so as to track a moving character that is projected on a screen and the apparent source of the sound will move across the screen along with the character. One may project the beam at a stationary or moving individual in an area in which other individuals are also positioned and the demodulated sound will be heard by that individual, largely to the exclusion of others. Similarly, one may project the beam into an area so that individuals who pass into the area will receive a message keyed to that location. For example, in an art gallery, messages keyed to individual paintings may be projected into the areas in front of the paintings.
With such useful applications for parametric sonic beam technology, one would expect it to have a wide commercial application. This has not been the case, however, and it appears that several factors have militated against commercial acceptance. For example, the transducer arrays that project the ultrasonic beams have heretofore been expensive to manufacture and characterized by low efficiency converting electrical energy into acoustical energy, resulting in bulky and cumbersome systems.
Moreover, the transducers have been characterized by a narrow bandwidth, making it difficult to compensate for distortion as discussed herein.
Another deficiency in prior systems has been the use of a relatively low ultrasonic carrier frequency, e.g., 40 kHz, which can result in modulation components whose frequencies are close to the upper limit of human audibility. Thus the intensities of these components can be such as to damage human hearing without the victims being aware of the high-intensity environment and thus being unaware of the harm to which they are subjected. Moreover, these components are well within the hearing range of household pets and can be very annoying or harmful to them as well. With inefficient transducers it is impractical to use higher frequencies, since atmospheric absorption of ultrasonic energy increases rapidly as a function of frequency.